1. Field of the Invention
The invention relates to an art for evaluating the specific surface area of electrode catalysts fixed to fuel cell electrodes, an art for evaluating performance of fuel cells, an electrode catalyst evaluated according to the arts, and a fuel cell having the electrode catalyst.
2. Description of Related Art
The performance of electrode catalysts is one of the factors crucial for the performance of fuel cells, and a number of arts for evaluating performance of electrode catalysts have been suggested. For example, there is known a process wherein a fuel cell is formed and then discharged for measurement of its output voltage or a cyclic voltammetry process (CV process) wherein an electrode catalyst is soaked in an electrolytic solution and the voltage is swept for measurement of current. Both of these processes electrochemically perform evaluation by supplying electricity to electrode catalysts.
A fuel cell, which has been studied and developed earnestly in recent years, is a stack of a plurality of unit cells. The unit cell is formed by having a membrane-electrode assembly (MEA) sandwiched between separators. In general, if a fuel cell as a stack of unit cells is expected to demonstrate desired performance, each of MEA's is required to have performance exceeding a desired level. Accordingly, if an electrode catalyst is to be evaluated after formation of a fuel cell, it is necessary to evaluate MEA's individually. Because the output current from the respective MEA's is negligible, the necessity to pay close attention to adhesion of metal ions makes the process of measurement troublesome. Furthermore, the evaluation of an electrode catalyst is possible only after formation of MEA's, which require troublesome work when being formed and can be adjusted in quest of desired characteristics only with great difficulty.
In general, an electrode catalyst for fuel cells, which has a noble metal such as platinum carried on a carbon carrier, is fixed onto an electrolytic membrane together with an electrolyte binder, thus forming an MEA. The carbon carrier generally has a particle size of 20 nm to 100 nm and is characterized in that carbon atoms (primary particles) securely adhere to one another to form a secondary particle (agglomerate). Platinum has a particle size of about 2 nm to 3 nm. On the other hand, since the particle size of the electrolytic binder is larger than the distance between the primary particles, the electrolytic binder cannot enter gaps between the primary particles. Consequently, part of the platinum cannot contribute to chemical reaction. According to the CV process wherein an electrode catalyst is soaked in an electrolytic solution for measurement, the particle size of the electrolytic solution used herein is smaller than the distance between primary particles, and the electrolytic solution also comes into contact with platinum that is out of contact with an electrolytic binder (platinum that is irrelevant to catalytic reaction). Accordingly, the performance of the electrode catalyst is evaluated, taking into account the platinum that is irrelevant to the reaction. As a result, it is difficult to evaluate performance of electrode catalysts (performance of fuel cells) with sufficient accuracy.